This invention relates generally to multi-stage centrifugal pumps, and, more particularly, to axially balanced multi-stage pumps having canned magnetic bearings at their outboard ends.
Axially balanced multi-stage pumps typically include a casing having an inlet port and an outlet port, an impeller shaft supported for rotation within the casing by external oil-lubricated bearings, a seal structure arranged between the casing and the impeller shaft, at each end of the impeller shaft and a plurality of impellers mounted on the impeller shaft. To balance the axial thrust imparted to the impeller shaft by the impellers during operation of the pump, the impellers are typically mounted back-to-back, wherein a first set of the impellers are arranged in a first direction and a second set of the impellers are arranged in a second direction, opposite the first direction.
In multi-stage pumps having the back-to-back impeller arrangement, one end of the pump is typically at a higher pressure than the other end. Consequently, the pump may still be imbalanced because the pressure of the pumped fluid acting on the seal structure at the high pressure end of the pump will be greater than that acting on the other seal structure at the low pressure end. To overcome this seal pressure imbalance, a seal pressure chamber may be provided at the high pressure end of the pump and placed in fluid communication with the low pressure end of the pump via a leakage return line to equalize the pressures at each end of the pump. A fluid flow restrictor may also be placed in the high pressure end to limit fluid leakage to the seal pressure chamber. Further details of this type of axially balanced multi-stage pump are set forth in U.S. Pat. No. 3,718,406 issued to Onal.
The axially balanced multi-stage centrifugal pump described above functions generally satisfactorily to accommodate the axial thrust being generated. However, it is nevertheless subject to drawbacks. In particular, the flow of pumped fluid through the leakage return line represents a significant loss of power and thus lowers pump efficiency. Additionally, the seals and oil lubricated bearings are among the higher maintenance items on the pump, resulting in increased repair costs and decreased overall reliability.
It should therefore be appreciated that there is still a need for a multi-stage centrifugal pump having increased efficiency and reliability and fewer maintenance problems. The present invention satisfies this need.